[charlie66]

For all you guys that stretch bolts, Arp says 63 ft lbs. on these or .0059" - .0063" if you want to stretch.

I'm getting between .004" - .0049" stretch at 70 ft lbs. Do I keep going higher on the torque until I reach what they're saying or is this good enough. It feels like it's a lot over the 63 ft lbs. as it is.

What do you guys feel about this?

Before this I've always just went for the 63 ft lbs. and never had a problem, but I figured I'd stretch them this time being its said to be more optimal for the full strength of the blot.

Thanks

[steve25]

The stretch is more important then the torque and even if your using the wrong lube stretch is stretch.
Keep adding torque until you get to .006”.

[performerrpm]

Originally Posted by steve25

The stretch is more important then the torque and even if your using the wrong lube stretch is stretch.
Keep adding torque until you get to .006”.

i guess when you put it that way...i should buy the stretch gauge and check this on all my current builds. ive just been doing the tq numbers...but with arps moly lube.

[ANDYA]

I ran into this recently with Crower rods while checking bearing clearances. The torque spec does not equal the stretch spec. First I torqued the bolts to spec and then checked the stretch. The bolts were not even close to the required stretch. Then I checked the clearances with a bore gauge and they were good. I tried tightening the bolts using a stretch gauge and it took a lot of extra torque just to get to the minimum stretch requirement. Seems like more than an additional 20 lbs of torque. Then I checked the clearances again and they were way too tight. It seems like going by the stretch method distorted the big end out of round and closed down the clearances. It seems to me that when Crower machined the big ends the caps were torqued to spec not stretched to spec. I spoke with a few Pontiac engine builders who agreed with me. When I put the motor together I’m going with torque. Strange thing is years ago I put together another motor using Crower rods. The torque almost equaled the required stretch. I only had to give the bolts an extra bump with a wrench to get the correct stretch.

[Dragncar]

I ran my 8740 bolts at 64lbs, no issues. The stretch deal, I know its the "correct" way to do it and all. But my buddy was building a sbf with Molnar rods and he did the stretch method. it took 92lbs to get the stretch Molnar wanted. He has no issues.
I did the torque and angle method and had no issues either.
92lbs would creep me out.

[Dragncar]

Originally Posted by ANDYA

I ran into this recently with Crower rods while checking bearing clearances. The torque spec does not equal the stretch spec. First I torqued the bolts to spec and then checked the stretch. The bolts were not even close to the required stretch. Then I checked the clearances with a bore gauge and they were good. I tried tightening the bolts using a stretch gauge and it took a lot of extra torque just to get to the minimum stretch requirement. Seems like more than an additional 20 lbs of torque. Then I checked the clearances again and they were way too tight. It seems like going by the stretch method distorted the big end out of round and closed down the clearances. It seems to me that when Crower machined the big ends the caps were torqued to spec not stretched to spec. I spoke with a few Pontiac engine builders who agreed with me. When I put the motor together I’m going with torque. Strange thing is years ago I put together another motor using Crower rods. The torque almost equaled the required stretch. I only had to give the bolts an extra bump with a wrench to get the correct stretch.

Crower did go by torque. All the stuff I got from them gave a torque value. 72 lbs for some of their rods I have.

[charlie66]

Yeah, I just got 4 rods installed at 78ft lbs. They all range from .004" to .0051 stretch. I just don't feel right going any more lol.

When I did my Carrillo Carr bolts 7 years ago, they ranged from 88 ft lbs. to 110 ft lbs.Lol

I only went through with that because they were telling me to. It didn't feel right.

[Elarson]

One of the big aerospace OEM's that I do work for, has a super slippery proprietary thread lube that takes half the torque as engine oil to get the proper stretch. Torque is highly dependent on the thread lube, surface finish, steady turning method, etc. Stretch is independent of all those variables.

Eric

[mgarblik]

To add to what Eric posted. IMO, it is critical to follow the connecting rod manufacturers recommendations for what lube to use. Even though many connecting rod companies use ARP rod bolts, many do NOT use ARP Ultra Torque lube when designing their tightening specs. Many specify CMD Extreme Pressure lube #3. That is a popular one. Some specify engine oil only. Some do use Ultra Torque lube. As Eric mentioned, the torque value can vary wildly based on the lube used. One of the super nice features of the modern electronic torque wrenches with angle is the ability to see what the final torque reading is once a set angle has been reached. I tested several sets of Molnar rods which specify torque angle for tightening. I measured the stretch, angle and final torque. Angle and stretch matched really close. Final torque was not nearly as accurate. +-15% on average. That doesn't mean your connecting rods are going to fail, but definitely not as accurate.

[Skip Fix]

And then throw in the variability of torque wrenches!

[charlie66]

Im using CMD Extreme Pressure lube #3

[Elarson]

And this is common knowledge but is important enough to restate; but torque is only valid if the nut is turning at a steady speed. As soon as it's allowed to stop turning, static friction is much higher than sliding friction.

Eric

[PAUL K]

The most important thing is to torque, stretch or angle the rod bolt to the same dimension the rod was honed. With that being said, stretching the bolt is done to get maximum performance from the bolt. "IF" you don't have a max effort build, you most likely will never need to worry about the rod bolt performing to its absolute limit. The downfall of stretching a rod bolt is, if it's ever over stressed it fails. JMHO unless you really have a grasp of what's needed for a build, you are by far better off following the instructions that came with the connecting rods, that's why they were sent with the rods. If something doesn't jive I would suggest you reach out to the guys that made them and explain what is going on.

[steve25]

I have done this in the past before I got my stretch gauge.
As stated in one of the above post a torque reading is only valid while the nut or bolt is moving!

If using the right lube what I use to do is set my calibration checked click type wrench to 2 lbs less then the rod bolts called for and then tried to focus on keeping it spinning until it clicks.

In this way at least if you go a hair too far your ok.

Another check to make is to pull the rods up to 80% of the torque that’s called for 3 times while
noting if the wrench moves more each time.

This seems to help when I first got my stretch gauge to compare changes
As I made checks between using the torque wrench and stretch method.

[charlie66]

I always snug the bolt until I feel it seat the rod and cap tight then in one motion pull it to the torque value I'm looking for.

Has anyone ever stretched an ARP 8740 bolt to spec and have to go to or more then 78ft lbs.?

[charlie66]

When I bought my Molnar power adders I was told to only stretch them. When I got them, they were torqued. Before I loosened them, I measured them. they were only stretched to .004" so they were honed to size at that stretched amount. So being they want you to go more I would have to believe that the extra torque wouldn't distort the bore. I have the motor running now for a year with no problems at all.

[Formulas]

Going from memory my Molnar rods i torqued to lower stretch limits with their spiffy lube using a deflecting beam and they were lower 80's FTLBs.... was anxious about how much torque it took but nothing says your there like a good stretch...

[charlie66]

My Molnar's took over 90 ft lbs

[Skip Fix]

Buddy torqued his SBC thinking they were 7/16" bolts when they were 3/8". Kicked them out the side!

[SD421]

Im going to be reassembling my motor soon. I too have ARP 8470 bolts but on RPM 4340 rods.
My machinist told me yesterday that unless its a race engine I can just torque them to 63 ft lbs. He did give me a packet of ARP lube.

So is the consensus still use a bolt stretcher gauge or just torque to spec?
This is my second run with these bolts.